Carbon fiber is employed as a fiber for reinforcing a composite material comprising a plastic usually called matrix resin owing to its excellent mechanical property, and is applied widely in various end uses including aerospace industry, sports goods industry, and other general industries.
A common method for manufacturing carbon fiber involves a process of producing precursor (also referred to as fiber production process), a process of converting the precursor into an oxidized fiber in an oxidative atmosphere at 200 to 300 deg. C. (hereinafter sometimes referred to as oxidative stabilization process), and a process of carbonizing the oxidized fiber in an inert atmosphere at 300 to 2,000 deg. C. (hereinafter sometimes referred to as carbonizing process). The oxidative stabilization and carbonizing processes are hereinafter sometimes collectively referred to as baking process. The process of producing precursor includes a drawing step where acrylic fiber is drawn with a draw ratio higher than that for an ordinary acrylic fiber. At the drawing step, acrylic fiber is apt to adhere to adjacent fiber strands, drawn unevenly under high draw ratio, and processed into nonuniform precursor. Such nonuniform precursor poses a problem, i.e., insufficient tenacity of resultant carbon fiber which is produced by baking the precursor. The baking process also poses another problem, i.e., fusing of single precursor fibers, which reduces the quality and grade of resultant carbon fiber.
For preventing the adhesion of single precursor fibers and the fusion of carbon fiber, a number of techniques to apply finishes to precursors have been suggested (refer to Patent References 1 and 2) and widely employed in industries, in which silicone finishes attaining low fiber-to-fiber wet friction at high temperature and excellent fiber detaching property, especially finishes comprising amino-modified silicones which cross-link on fiber to improve the heat resistance of the fiber, are used. Those silicone finishes, however, sometimes failed to produce carbon fiber having sufficient tenacity.